White conductive primer coating composition and method of forming multilayered coating film

ABSTRACT

A white conductive primer coating composition comprising (a) 100 parts by total weight of a chlorinated polyolefin resin with a chlorine content of 10 to 40 wt. % and at least one modifier resin selected from the group consisting of acrylic resins, polyester resins and polyurethane resins, (b) 5 to 50 parts by weight of a crosslinking agent, and (c) 10 to 200 parts by weight of a white conductive powder comprising white inorganic pigment particles having a tin dioxide covering layer containing tungsten; and a method of forming a multilayer coating film using the composition.

TECHNICAL FIELD

The present invention relates to a white conductive primer coatingcomposition and a method of forming a multilayer coating film.

BACKGROUND ART

Automotive bumpers and like plastic substrates are generally coated byspray coating, such as air spray coating or airless spray coating. Inrecent years, however, electrostatic coating, which shows excellentdeposition efficiency and thus emits only a small amount ofenvironmentally harmful substances, has been finding wider application.

Since plastic substrates generally have high electrical resistance(usually about 10¹² to about 10¹⁶ Ω/sq.), it is extremely difficult toelectrostatically apply a paint to plastic substrate surfaces directly.Therefore, before electrostatic application, conductivity is imparted toplastic substrates themselves or their surfaces so that the substrateshave a surface electrical resistance below 10⁹ Ω/sq.

For example, before electrostatic application of a paint to a plasticsubstrate, a conductive primer may be applied to impart conductivity tothe substrate. A coating material containing a resin component andconductive filler is conventionally used as such a conductive primer.

Particles of conductive carbon, metals, conductive metal oxides, etc.have been heretofore used as conductive fillers. The form or shape ofsuch conductive filler particles is usually a powder, needles, fibers,spheres or the like.

When a carbon powder or carbon fibers are added to a coating material asa conductive filler, although a relatively small amount can impartconductivity, the resulting coating layer has reduced whiteness, i.e.,reduced brightness, and thus affects the color properties, such asbrightness, of the upper coating layer to be formed thereon.

Metal powders have high conductivity, but need to be added in largeamounts, since the particles of metal powders need to be in contact withone another to form an electrical conduction path in a coating layer.Thus, use of a metal powder as a conductive filler impairs the whitenessof the coating layer and stability of the coating material.

Japanese Unexamined Patent Publication No. 2001-311047 proposes aconductive coating composition containing a specific sulfonium saltcompound. Reportedly, this composition can make plastic substratessuitable for electrostatic coating, and has little influence on thecolor tone of the upper coating layer when forming a multilayer coatingfilm.

However, the sulfonium salt compound in this coating compositionadversely affects the environment when baking the coating of thecomposition or recycling the coated plastic substrates. Further, thecoating film formed from the composition has insufficient whiteness.

Japanese Unexamined Patent Publication No. 2002-179948 proposes aconductive resin composition prepared by blending, with a resin, a whiteconductive powder comprising white inorganic pigment particles having ontheir surfaces a tin dioxide covering layer containing tungsten.

However, this publication does not describe a specific formulation ofthe conductive resin composition, and mentions nothing about primercoating compositions. Thus, it cannot be known what formulation of thewhite conductive powder, when used, gives a conductive primer coatingcomposition that can form, on a plastic substrate, a coating layer withhigh brightness and sufficient conductivity for electrostaticapplication of an upper coating layer thereto.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a white conductiveprimer coating composition that can form a coating layer with sufficientconductivity and high brightness on a plastic substrate.

Another object of the present invention is to provide a method offorming a multilayer coating film on a plastic substrate using theprimer coating composition.

The present inventors conducted extensive research to develop a whiteconductive primer coating composition that can form a coating layer withsufficient conductivity and high brightness on a plastic substrate.

As a result, the inventors found that when a white conductive powdercomprising white inorganic pigment particles having on their surfaces atin dioxide covering layer containing tungsten is blended as aconductive filler with a specific chlorinated polyolefin resin, specificmodifier resin and crosslinking agent, the resulting primer coatingcomposition can impart sufficient conductivity to plastic substrates,thus allowing another coating composition to be electrostaticallyapplied over the coating layer of the primer coating composition; andthe coating layer of the primer coating composition has high brightness.The present invention was accomplished based on these new findings.

The present invention provides the following white conductive primercoating compositions and methods of forming a multilayer coating filmusing the composition.

1. A white conductive primer coating composition comprising:

(a) 100 parts by total weight of a chlorinated polyolefin resin with achlorine content of 10 to 40 wt. %, and at least one modifier resinselected from the group consisting of acrylic resins, polyester resinsand polyurethane resins;

(b) 5 to 50 parts by weight of a crosslinking agent; and

(c) 10 to 200 parts by weight of a white conductive powder comprisingwhite inorganic pigment particles having on their surfaces a tin dioxidecovering layer containing tungsten.

2. A primer coating composition according to item 1, wherein, in thecomponent (a), the proportions of the chlorinated polyolefin resin andmodifier resin are 10 to 90 wt. % and 90 to 10 wt. %, respectively, ofthe total of these resins.

3. A primer coating composition according to item 1, wherein the whiteconductive powder (c) comprises white inorganic pigment particles havinga tin dioxide layer on their surfaces and a tin dioxide covering layercontaining tungsten on the tin dioxide layer.

4. A primer coating composition according to item 1, wherein, in thewhite conductive powder (c), the proportion of the tungsten-containingtin dioxide covering layer is 3 to 150 wt. % on a tin dioxide basis,relative to the weight of white inorganic pigment.

5. A primer coating composition according to item 4, wherein, in thetungsten-containing tin dioxide covering layer in the white conductivepowder (c), the proportion of tungsten is 0.1 to 20 wt. % relative tothe weight of tin dioxide.

6. A primer coating composition according to item 1, further comprising(d) up to 200 parts by weight of a white pigment.

7. A primer coating composition according to item 1, the compositionbeing capable of forming a coating with a lightness (L* value) of 80 ormore as determined according to the L*a*b* color system defined in JIS Z8729, by being applied to a plastic substrate and cured by heating.

8. A primer coating composition according to item 1, the composition,when formed into an uncured or cured coating applied on a plasticsubstrate, having a surface electrical resistance of less than 10⁹ Ω/sq.

9. A primer coating composition according to item 1, which is an aqueouscoating composition.

10. A 3-coat 1-bake method of forming a multilayer coating film, themethod comprising the steps of:

(1) applying a white conductive primer coating composition according toitem 1 to a plastic substrate;

(2) electrostatically applying a colored base coating composition on theuncured coating layer of the primer coating composition;

(3) electrostatically applying a clear coating composition on theuncured coating layer of the base coating composition; and then

(4) curing by heating the three-layer coating comprising the primercoating composition, colored base coating composition and clear coatingcomposition.

11. A 3-coat 2-bake method of forming a multilayer coating film, themethod comprising the steps of:

(1) applying a white conductive primer coating composition according toitem 1 to a plastic substrate, followed by curing by heating;

(2) electrostatically applying a colored base coating composition on thecured coating layer of the primer coating composition;

(3) electrostatically applying a clear coating composition on theuncured coating layer of the base coating composition; and then

(4) curing by heating the two-layer coating comprising the colored basecoating composition and clear coating composition.

White Conductive Primer Coating Composition

The white conductive primer coating composition of the present inventionis an aqueous or organic solvent-based coating composition comprisingspecific amounts of (a) a mixture of a chlorinated polyolefin resin witha chlorine content of 10 to 40 wt. %, and at least one modifier resinselected from the group consisting of acrylic resins, polyester resinsand polyurethane resins; (b) a crosslinking agent; and (c) a whiteconductive powder comprising white inorganic pigment particles having ontheir surfaces a tin dioxide covering layer containing tungsten.

Chlorinated Polyolefin Resin/Modifier Resin Mixture (a)

A mixture of a chlorinated polyolefin resin and specific modifierresin(s) is used as a resin component of the white conductive primercoating composition of the present invention. The chlorinated polyolefinresin is used to improve the adhesion of the coating layer, and themodifier resin(s) is used to adjust the flexibility, stiffness and otherproperties of the coating layer and improve the film-forming properties.

Chlorinated Polyolefin Resin

The chlorinated polyolefin resin is a chlorination product of apolyolefin. Examples of usable base polyolefins include radicalhomopolymers and copolymers of at least one olefin selected fromethylene, propylene, butene, methylbutene, isoprene, etc.; and radicalcopolymers of such olefins with vinyl acetate, butadiene, acrylic ester,methacrylic ester, etc. The chlorinated polyolefin may usually have aweight average molecular weight of about 30,000 to about 200,000,especially about 50,000 to about 150,000.

The chlorine content of the chlorinated polyolefin resin is about 10 toabout 40 wt. %. Chlorine contents within the above range do not impairthe solubility in solvents, and thus the coating composition can besufficiently atomized during spray coating. Further, such chlorinecontents do not reduce the solvent resistance of the resulting coatinglayer. A preferable chlorine content is about 12 to about 35 wt. %.

Examples of preferable chlorinated polyolefin resins include chlorinatedpolyethylene, chlorinated polypropylene, chlorinated ethylene-propylenecopolymers, chlorinated ethylene-vinyl acetate copolymers, etc. Alsousable are resins obtained by graft polymerization of chlorinatedpolyolefins with polymerizable monomers.

Examples of polymerizable monomers usable for such graft polymerizationinclude alkyl (meth)acrylates, alkoxyalkyl (meth)acrylates, glycidyl(meth)acrylates, adducts of glycidyl (meth)acrylates with monocarboxylicacids, hydroxyalkyl (meth)acrylates, acrylic acid, methacrylic acid,etc. The proportion of such polymerizable monomer(s) to be used is notlimited as long as gelation does not occur, and is preferably about 10to about 80 wt. %, and more preferably about 30 to about 60 wt. %,relative to the chlorinated polyolefin.

When the primer coating composition of the present invention is aqueous,in order to make the chlorinated polyolefin resin water-dispersible, oneor more hydrophilic monomers, such as a polymerizable unsaturateddicarboxylic acid or an anhydride thereof, may be graft-polymerized tothe chlorinated polyolefin by a known method.

Polymerizable unsaturated dicarboxylic acids and anhydrides thereof arecompounds having one polymerizable unsaturated bond and two or morecarboxy groups per molecule, or anhydrides thereof. Examples includemaleic acid and its anhydride, itaconic acid and its anhydride,citraconic acid and its anhydride, etc. The proportion of suchhydrophilic monomer(s) is preferably about 1 to about 60 wt. %, and morepreferably about 1.5 to about 40 wt. %, relative to the chlorinatedpolyolefin.

The graft polymerization of the monomer(s) to the chlorinated polyolefinresin can be performed by a known method. The modified chlorinatedpolyolefin obtained by using the above-mentioned proportion ofpolymerizable unsaturated dicarboxylic acid or anhydride thereof usuallyhas a saponification value of about 10 to about 60 mg KOH/g, especiallyabout 20 to about 50 mg KOH/g.

In a chlorinated polyolefin resin to which a polymerizable unsaturateddicarboxylic acid or an anhydride thereof has been graft-polymerized asdescribed above, it is preferable that part or all of the carboxy groupsbe neutralized with an amine compound to render the chlorinatedpolyolefin resin water-soluble or water-dispersible.

Examples of amine compounds include triethylamine, tributylamine,dimethylethanolamine, triethanolamine and other tertiary amines;dimethylamine, dibutylamine, diethanolamine and other secondary amines;etc. To impart water solubility or water dispersibility, such aminecompounds can be used in combination with surfactants.

Modifier Resin

In the resin component of the white conductive primer coatingcomposition of the present invention, a modifier resin is used incombination with the chlorinated polyolefin resin for adjusting theflexibility, stiffness, etc. of the coating layer, improving thefilm-forming properties, and other purposes. At least one memberselected from the group consisting of acrylic resins, polyester resinsand polyurethane resins is used as the modifier resin.

In view of the balance between adhesion of the coating layer andmodification effects, the proportions of the chlorinated polyolefinresin and the modifier resin are preferably about 10 to about 90 wt. %and about 90 to about 10 wt. %, respectively, of the total of theseresins. More preferable proportions are about 20 to 70 wt. % of thechlorinated polyolefin resin and about 80 to about 30 wt. % of themodifier resin.

Hydroxy-containing acrylic resins are preferable as acrylic resins foruse as modifier resins. When the primer coating composition of thepresent invention is aqueous, it is preferable to use ahydroxy-containing acrylic resin containing, as well as hydroxy groups,carboxy groups to impart solubility or dispersibility in water,crosslinkability, etc.

Hydroxy-containing acrylic resins can be obtained by polymerizing ahydroxy-containing monomer, alkyl (meth)acrylate monomer, and optionallyother monomers, by a known polymerization method, such as a solutionpolymerization method or the like.

Hydroxy-containing monomers are compounds having a hydroxy group andpolymerizable unsaturated group. Examples thereof include hydroxyethyl(meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl(meth)acrylate, other monoesters of (meth)acrylic acid with C₂₋₁₀ diols,etc.

Examples of alkyl (meth)acrylate monomers include methyl (meth)acrylate,ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, hexyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate,stearyl (meth)acrylate, other monoesters of (meth)acrylic acid withC₁₋₂₀ monohydric alcohols, etc.

The other monomers are compounds that are other than hydroxy-containingmonomers and alkyl (meth)acrylate monomers and have a polymerizableunsaturated bond. Examples of such monomers include (meth)acrylic acid,maleic acid and other carboxy-containing monomers; glycidyl(meth)acrylate and other epoxy-containing monomers; (meth)acrylamide,acrylonitrile, styrene, vinyl acetate, vinyl chloride, etc.

It is usually suitable that such hydroxy-containing acrylic resins havea hydroxy value of about 10 to about 100 mg KOH/g, and preferably about50 to about 90 mg KOH/g; an acid value of about 10 to 100 mg KOH/g, andpreferably about 30 to about 60 mg KOH/g; and a number average molecularweight of about 2,000 to about 100,000, and preferably about 3,000 toabout 50,000.

Polyester resins for use as modifier resins can usually be obtained byan esterification reaction of a polybasic acid with a polyhydricalcohol. Polybasic acids are compounds having two or more carboxy groupsper molecule (their anhydrides are also included); and polyhydricalcohols are compounds having two or more hydroxy groups per molecule.Conventionally used polybasic acids and polyhydric alcohols are usable.Also usable are those modified with a monobasic acid, higher fatty acid,oil component, etc.

Such polyester resins may have hydroxy groups, which can be introducedby using a divalent alcohol in combination with a trivalent or higheralcohol. The polyester resins may have carboxy groups as well as hydroxygroups, and it is usually suitable that the polyester resins have aweight average molecular weight of 1,000 to about 100,000, andpreferably about 1,500 to about 70,000.

Preferable polyurethane resins for use as modifier resins are thoseobtained by reacting a polyhydroxy compound, a polyisocyanate compoundand a compound having one active hydrogen atom per molecule. It isusually suitable that the polyurethane resins have a number averagemolecular weight of about 400 to about 10,000, and preferably about1,000 to about 4,000.

It is preferable that the polyhydroxy compound have at least twoalcoholic hydroxy groups per molecule; a number average molecular weightof about 50 to about 8,000, and particularly about 50 to about 6,000;and a hydroxy equivalent of about 25 to about 4,000, and particularlyabout 25 to about 3,000. Examples of such compounds include polyhydricalcohols; various polyester polyols and polyether polyols conventionallyused for producing polyurethane resins; mixtures thereof; etc.

Polyisocyanate compounds are compounds having two or more, andpreferably two or three, isocyanate groups per molecule. Examples ofsuch compounds include aliphatic polyisocyanate compounds, alicyclicpolyisocyanate compounds, aromatic polyisocyanate compounds, and othercompounds conventionally used for producing polyurethane resins.

The compound having one active hydrogen atom per molecule is used forblocking isocyanate groups in the polyisocyanate compound. Examples ofsuch compounds include methanol, ethanol, diethylene glycol monobutylether and other monohydric alcohols; acetic acid, propionic acid andother monocarboxylic acids; ethyl mercaptan and other monothiols;diethylenetriamine, monoethanolamine and other primary amines;diethylamine and other secondary amines; methyl ethyl ketoxime and otheroximes; etc.

When the primer coating composition of the present invention is aqueous,a hydrophilic polyurethane resin that can be dissolved or dispersed inwater is preferable as a polyurethane resin for use as a modifier resin.

Hydrophilic polyurethane resins can be obtained, for example, byextending and emulsifying, after or while being neutralized, a urethaneprepolymer obtained by reacting, by a one-shot or multistage process, analiphatic and/or alicyclic diisocyanate, diol with a number averagemolecular weight of about 500 to about 5,000, a low-molecular-weightpolyhydroxyl compound and a dimethylolalkanoic acid. Particularlypreferable is a self-emulsifiable urethane resin with a mean particlediameter of about 0.001 to about 1 μm, obtained by distilling off someor all of the organic solvent used in the production process.

Commercial products of polyurethane resins can be used, including, forexample, “Takelac W610” (a tradename of Takeda Pharmaceutical Co.,Ltd.), “Neorez R960” (a tradename of Zeneca Resins Ltd.), “SanpreneUX-5100A” (a tradename of Sanyo Chemical Industries, Ltd.), etc.

Crosslinking Agent (b)

The white conductive primer coating composition of the present inventioncontains a crosslinking agent together with the above resin component toimprove the film performance characteristics, such as water resistance,and is used as a thermosetting coating composition.

Usable crosslinking agents include polyisocyanate compounds withunreacted isocyanate groups; blocked polyisocyanate compounds obtainedby blocking isocyanate groups of polyisocyanate compounds with blockingagents; melamine resins; epoxy resins; carbodiimide resins; oxazolinecompounds; etc.

Examples of polyisocyanate compounds with unreacted isocyanate groupsinclude tolylene diisocyanate (TDI), 4,4′-diphenylmethane diisocyanate(MDI), xylylene diisocyanate (XDI), metaxylylene diisocyanate (MXDI) andother aromatic diisocyanates; hexamethylene diisocyanate (HDI) and otheraliphatic diisocyanates; isophorone diisocyanate (IPDI), hydrogenatedMDI and other alicyclic diisocyanates; such diisocyanate compounds in afixed and less toxic form; biurets, uretdiones, isocyanurates andadducts of such diisocyanate compounds; relatively low-molecular-weighturethane prepolymers; and other polyisocyanate compounds.

When the primer coating composition of the present invention is aqueous,it is preferable to use a polyisocyanate compound as hydrophilized.Polyisocyanate compounds can be hydrophilized, for example, byintroducing at least one hydrophilic group into the compounds, which arethen neutralized with a neutralizing agent. Examples of usablehydrophilic groups include carboxy groups, sulfonic acid groups,tertiary amino groups and the like, and examples of usable neutralizingagents include dimethylol propionic acid and like hydroxycarboxylicacids, ammonia, tertiary amine and the like. A surfactant may also bemixed with polyisocyanate compounds for emulsification, to prepareso-called self-emulsifiable polyisocyanate compounds.

Commercial products of hydrophilic polyisocyanate compounds are usable,including, for example, “Bayhydur 3100” (a tradename of Sumika BayerUrethane Co., Ltd., a hydrophilized isocyanurate of hexamethylenediisocyanate) and the like.

Blocked polyisocyanate compounds can be obtained by blocking isocyanategroups of a polyisocyanate compound with a blocking agent.

Usable blocking agents include ε-caprolactam, γ-butyrolactam and otherlactam compounds; methyl ethyl ketoxime, cyclohexanone oxime and otheroxime compounds; phenol, para-t-butylphenol, cresol and other phenolcompounds; n-butanol, 2-ethylhexanol and other aliphatic alcohols;phenylcarbinol, methylphenylcarbinol and other aromatic alkyl alcohols;ethylene glycol monobutyl ether and other ether alcohol compounds; etc.

Blocked polyisocyanate compounds may be obtained by blocking apolyisocyanate compound with a blocking agent and then dispersing theblocked product in water using a suitable emulsifier and/or protectivecolloid agent, since such a blocked product is usually hydrophobic.

Examples of melamine resins include methylolated melamine resinsobtained by reacting melamines with formaldehyde; partially or fullyetherified melamine resins obtained by reacting methylolated melamineresins with C₁₋₁₀ monohydric alcohols; etc. Such melamine resins maycontain imino groups. The melamine resins may be hydrophobic orhydrophilic. Especially suitable are hydrophilic, methyl-etherifiedmelamine resins with a low degree of condensation and a number molecularweight less than about 3,000, particularly about 300 to about 1,500.Examples of commercial products of such hydrophilic melamine resinsinclude “Cymel 303”, “Cymel 325” (tradenames of Cytec Industries, Inc.),etc.

Epoxy resins have two or more epoxy groups per molecule, and areeffective for crosslinking and curing chlorinated polyolefins, acrylicresins, polyester resins, polyurethane resins and like resins, allhaving carboxy groups.

Examples of epoxy resins include copolymers of epoxy-containingpolymerizable monomers and polymerizable vinyl monomers. Examples ofepoxy-containing polymerizable monomers include glycidyl acrylate,glycidyl methacrylate, methyl glycidyl acrylate, methyl glycidylmethacrylate, etc. Polymerizable vinyl monomers are those other thanepoxy-containing polymerizable monomers, and include, for example, alkyl(meth)acrylates, acrylonitrile, styrene, vinyl acetate, vinyl chloride,etc. The copolymerization reaction of such monomers can be performed bya known process, and it is preferable that the obtained polymer have anepoxy equivalent of about 200 to about 2,800, and especially about 300to about 700, and a number average molecular weight of about 3,000 toabout 100,000, and especially about 4,000 to about 50,000.

Also usable as the crosslinking agent are epoxy resins such asglycidyl-etherified bisphenols; hydrogenated products thereof;glycidyl-etherified aliphatic polyhydric alcohols; glycidyl ester-typeepoxy resins; alicyclic epoxy resins; etc. Such epoxy resins preferablyhave a molecular weight of about 250 to about 20,000, and especiallyabout 300 to about 5,000.

Commercial products of carbodiimide resins can be used, including, forexample, “Carbodilite E-01”, “Carbodilite E-02” (tradenames of NisshinboIndustries, Inc.), etc.

Oxazoline compounds are hydrophilic compounds effective for crosslinkingand curing chlorinated polyolefins, acrylic resins, polyester resins,polyurethane resins and like resins, all having carboxy groups. Usableas such hydrophilic oxazoline compounds are a commercial product“Epocros WS-500” (a tradename of Nippon Shokubai Co., Ltd.) and thelike.

White Conductive Powder (c)

The conductive primer coating composition of the present inventioncontain as a conductive filler a white conductive powder (c) comprisingwhite inorganic pigment particles having on their surfaces a tin dioxide(SnO₂) covering layer containing tungsten.

The shape of the white inorganic pigment particles may be, for example,granules, approximate spheres, spheres, needles, fibers, columns,cylinders, spindles, plates and the like. Among these, particles in theshape of granules, approximate spheres or spheres are preferable, sincethese usually have an aspect ratio less than 3, and plastic substratescoated with a primer coating composition prepared using such particlesare highly safe to the human body when being recycled.

Further, the white inorganic pigment particles preferably have a meanparticle diameter of about 0.05 to about 2.0 μm, and more preferablyabout 0.1 to about 1.0 μm, in view of dispersibility and other factors.

The type of white inorganic pigment particles is not limited as long asthe particles do not undergo a change in shape, decomposition or thelike. Examples of usable particles include those of titanium dioxide,aluminium oxide, silicon dioxide, zinc oxide, barium sulfate, zirconiumoxide, alkali metal titanates, muscovite, etc.

As the white conductive powder (c), for example, the following can beused: (1) a powder comprising white inorganic pigment particles havingon their surfaces a tin dioxide (SnO₂) covering layer containingtungsten; (2) a powder comprising white inorganic pigment particleshaving on their surfaces a tin dioxide (SnO₂) layer as an adhesive lowerlayer, and a tin dioxide (SnO₂) covering layer containing tungsten overthe lower layer; etc.

In white conductive powders (1) and (2), the proportion of thetungsten-containing tin dioxide covering layer is preferably about 3 toabout 150 wt. % calculated as tin dioxide, relative to the weight ofwhite inorganic pigment. A covering layer in a proportion within theabove range can impart sufficient conductivity and sufficient brightnessto the coating layer of the primer coating composition. More preferably,the proportion of the tungsten-containing tin dioxide covering layer isabout 10 to about 120 wt. %.

Further, in the white conductive powder (c), it is preferable that thetungsten-containing tin dioxide covering layer contains tungsten in aproportion of about 0.1 to about 20 wt. %, relative to the weight of tindioxide in the covering layer. Tungsten in a proportion within the aboverange can impart sufficient conductivity to the coating layer of theprimer coating composition, and does not substantially affect thebrightness of the coating layer. More preferably, the proportion oftungsten is about 0.5 to about 15 wt. %.

In white conductive powder (2), the proportion of the lower tin dioxidelayer is preferably about 1 to about 15 wt. %, calculated as tindioxide, relative to the weight of white inorganic pigment. A lower tindioxide layer in a proportion within the above range can improve theadhesion between the white inorganic pigment particles and thetungsten-containing upper tin dioxide covering layer. More preferably,the proportion of the lower tin dioxide layer is about 3 to about 15 wt.%.

White conductive powder (1) can be prepared, for example, as follows.First, the surfaces of white inorganic pigment particles are coveredwith a tin salt or stannate containing a tungsten-containing compoundby: adding, sequentially or simultaneously, an aqueous tin salt orstannate solution and a tungsten-containing compound dissolved in analkali or acid to an aqueous suspension of the pigment particles; oradding, sequentially or simultaneously, a tungsten-containing compounddissolved in an aqueous tin salt or stannate solution, and an alkali oracid, to an aqueous suspension of the pigment particles; or other suchprocesses. A suitable pH during this covering process is about 2 toabout 9. The coated particles are collected by filtration, dried andheat-treated, to thereby obtain a conductive powder in which thesurfaces of the pigment particles are covered with tungsten-containingtin dioxide. The heat treatment is performed preferably in anon-oxidizing atmosphere at about 400 to about 900° C.

White conductive powder (2) can be prepared, for example, as follows.First, the surfaces of white inorganic pigment particles are coveredwith a tin salt or stannate, by adding, sequentially or simultaneously,an aqueous tin salt or stannate solution and an alkali or acid, to anaqueous suspension of the pigment particles, or other such processes. Asuitable pH during the covering process is about 2 to about 9. Thecovered particles are collected by filtration and dried, and then thetin salt or stannate layer is further covered with a tin salt orstannate containing a tungsten-containing compound by the same procedureas for the preparation of white conductive powder (1). Subsequently, thecoated particles are collected by filtration, dried and heat-treated, tothereby obtain a conductive powder in which the pigment particles have atin dioxide layer on their surfaces as an adhesive lower layer, and atin dioxide covering layer containing tungsten over the lower layer. Theheat treatment is performed preferably in a non-oxidizing atmosphere atabout 400 to about 900° C.

Usable tin salts include, for example, tin chloride, tin sulfate, tinnitrate, etc. Usable stannates include, for example, sodium stannate,potassium stannate, etc.

Examples of tungsten-containing compounds include tungstates,metatungstates, paratungstates, tungsten compounds, etc. Examples oftungstates include ammonium tungstate, potassium tungstate, sodiumtungstate, etc. Examples of metatungstates include ammoniummetatungstate, potassium metatungstate, sodium metatungstate, etc.Examples of paratungstates include ammonium paratungstate, potassiumparatungstate, sodium paratungstate, etc. Examples of tungsten compoundsinclude tungsten oxychloride and the like.

Examples of alkalis include sodium hydroxide, potassium hydroxide,sodium carbonate, potassium carbonate, ammonium carbonate, aqueousammonia, ammonia gas, etc. Examples of acids include hydrochloric acid,sulfuric acid, nitric acid, acetic acid, etc.

White Pigment (d)

The white conductive primer coating composition may further contain (d)a white pigment to improve the whiteness of the coating layer to beobtained.

Usable as the white pigment (d) is, for example, titanium dioxide(rutile titanium dioxide, anatase titanium dioxide or the like), whitelead, zinc white, zinc sulfide, lithopone, etc. Among these, titaniumdioxide is preferable from the viewpoint of chemical resistance andwhiteness. More preferable is rutile titanium dioxide with a meanparticle diameter of about 0.05 to about 2.0 μm, and especially about0.1 to about 1.0 μm.

Formulation of White Conductive Primer Coating Composition

The white conductive primer coating composition of the present inventioncomprises (a) 100 parts by total weight of a chlorinated polyolefinresin with a chlorine content of 10 to 40 wt. %, and at least onemodifier resin selected from the group consisting of acrylic resins,polyester resins and polyurethane resins; (b) about 5 to about 50 partsby weight of a crosslinking agent; (c) about 10 to about 200 parts byweight of a white conductive powder comprising white inorganic pigmentparticles having on their surfaces a tin dioxide covering layercontaining tungsten.

The use of about 5 to about 50 parts by weight of the crosslinking agent(b) results in a coating composition with sufficient curability, andimproves film performance characteristics, such as water resistance andthe like.

The use of about 10 to about 200 parts by weight of the white conductivepowder (c) can impart sufficient conductivity to the coating layer ofthe primer coating composition, so that another coating composition canbe electrostatically applied over the coating layer. Further, such anamount of the white conductive powder does not impair the stability ofthe coating composition, and achieves excellent brightness, finishedappearance and other characteristics of the coating layer.

The amount of the crosslinking agent (b) is preferably about 10 to about45 parts by weight per 100 parts by weight of the total solids contentof the resin component (a). The amount of the white conductive powder(c) is preferably about 50 to about 180 parts by weight per 100 parts byweight of the total solids content of the resin component (a).

As described above, the white conductive primer coating composition ofthe present invention may further comprise a white pigment (d) toimprove the whiteness of the coating layer. The amount of the whitepigment (d) is usually about 200 parts by weight or less, preferablyabout 20 to about 180 parts by weight, and more preferably about 30 toabout 130 parts by weight, per 100 parts by weight of the total solidscontent of the resin component (a).

The white conductive primer coating composition of the present inventioncan be prepared by dissolving or dispersing the above-mentionedcomponents in an organic solvent, water or a mixture thereof by a knownmethod to adjust the solids content to about 15 to about 60 wt. %. Theprimer coating composition of the present invention may be an organicsolvent-based or aqueous composition, and to achieve low VOC and otheradvantages, is preferably an aqueous white conductive primer coatingcomposition.

As an organic solvent, the organic solvent used for preparation of eachcomponent may be used, or another organic solvent may be added asrequired.

Organic solvents usable in the composition of the present inventioninclude, for example, methyl ethyl ketone, methyl isobutyl ketone andother ketone solvents; ethyl acetate, butyl acetate and other estersolvents; ethylene glycol monobutyl ether and other ether solvents;isopropyl alcohol, n-butyl alcohol, isobutyl alcohol and other alcoholsolvents; n-heptane, n-hexane and other aliphatic hydrocarbon solvents;toluene, xylene and other aromatic hydrocarbon solvents; other solventssuch as N-methylpyrrolidone; etc.

Process for Applying White Conductive Primer Coating Composition

Various plastic substrates can be advantageously used as substrates tobe coated with the white conductive primer coating composition of thepresent invention.

Examples of usable plastic substrates include, but are not limited to,various plastic members for use in bumpers, spoilers, grilles, fendersand other automotive exterior panel parts, electrical home applianceexterior panel parts, etc.

The material of the plastic substrate is preferably, but not limited to,a polyolefin obtained by polymerizing at least one C₂₋₁₀ olefin, such asethylene, propylene, butylene, hexene or the like. Polycarbonates, ABSresins, urethane resins, nylon and other materials are also usable. Suchplastic substrates may be subjected to, as required, degreasing,water-washing and/or other treatments beforehand, by known methods.

The white conductive primer coating composition can be advantageouslyapplied, usually after being adjusted to a viscosity of about 12 toabout 18 seconds/Ford cup #4/20° C., to a plastic substrate surface byair spray coating, airless spray coating, dip coating or like coatingmethod. The composition is applied to a thickness of about 5 to about 50μm (when cured), and preferably about 10 to about 45 μm (when cured).

The applied primer coating composition is set or preheated, or cured byheating, to thereby obtain an uncured or cured coating layer thatusually has a surface electrical resistance of less than 10⁹ Ω/sq. Witha surface electrical resistance of less than 10⁹ Ω/sq., the coatinglayer can be electrostatically coated with other coating compositions,such as a colored coating composition and/or clear coating composition.

Known heating methods can be used for preheating or curing by heatingthe coating layer of the white conductive primer coating composition.Examples of usable methods include air blowing, infrared heating,far-infrared heating, induction heating, dielectric heating, etc. Theplastic substrate may be heated as required.

The aqueous white primer coating composition of the present inventioncan form a coating layer with high whiteness, i.e., a lightness (L*value) of 80 or more according to the L*a*b* color system as defined inJIS Z 8729. The lightness is a value measured as follows. The coatingcomposition (A) is applied to a plastic substrate by spray coating to athickness of about 20 μm (when cured), and cured by heating at about 80to about 120° C. for about 20 to about 40 minutes, and the lightness (L*value) of the obtained coating layer is measured using a colorimeter.Commercial calorimeters can be used, including, for example, “ColorComputer SM-7” (a tradename of Suga Test Instruments Co., Ltd.).

The following 3-coat 1-bake method I and 3-coat 2-bake method II, whichuse the white conductive primer coating composition of the presentinvention, can form multilayer coating films with bright color tones onplastic substrate surfaces.

Multilayer Coating Film Forming Method I

A 3-coat 1-bake method for forming a multilayer coating film, comprisingthe steps of:

-   (1) applying the white conductive primer coating composition of the    present invention to a plastic substrate, usually to a thickness of    about 5 to about 50 μm (when cured), and preferably about 10 to    about 40 μm (when cured);-   (2) electrostatically applying a colored base coating composition to    the uncured coating layer of the primer coating composition, usually    to a thickness of about 5 to about 30 μm (when cured), and    preferably about 10 to 25 μm (when cured);-   (3) electrostatically applying a clear coating composition to the    uncured coating layer of the base coating composition, usually to a    thickness of about 5 to about 50 μm (when cured), and preferably    about 10 to about 40 μm (when cured); and then-   (4) simultaneously curing by heating the three layers of the primer    coating composition, colored base coating composition and clear    coating composition.

In method I, the primer coating composition, colored coating compositionand clear coating composition, after being applied, may be set orpreheated as required. Setting is usually effected by allowing theapplied composition to stand at room temperature for about 1 to about 20minutes. Preheating is usually performed at about 40 to about 120° C.,for about 1 to about 20 minutes.

The three-layer coating comprising the primer coating composition,colored base coating composition and clear coating composition canusually be cured by heating at about 60 to about 140° C., for about 10to about 60 minutes. The curing is preferably performed by heating atabout 80 to about 120° C., for about 10 to about 40 minutes.

Multilayer Coating Film Forming Method II

A 3-coat 2-bake method of forming a multilayer coating film, comprising:

-   (1) applying the white conductive primer coating composition of the    present invention to a plastic substrate, usually to a thickness of    about 5 to about 50 μm (when cured), and preferably about 10 to    about 40 μm (when cured), followed by curing by heating;-   (2) electrostatically applying a colored base coating composition to    the cured coating layer of the primer coating composition, usually    to a thickness of about 5 to about 30 μm (when cured), and    preferably about 10 to about 25 μm (when cured);-   (3) electrostatically applying a clear coating composition to the    uncured coating layer of the base coating composition, usually to a    thickness of about 5 to about 50 μm (when cured), and preferably    about 10 to about 40 μm (when cured); and then-   (4) simultaneously curing by heating the two layers of the colored    base coating composition and clear coating composition.

In method II, the primer coating composition is usually cured by heatingat about 60 to about 140° C., for about 10 to about 60 minutes. Thecuring is preferably performed by heating at about 80 to about 120° C.,for about 10 to about 40 minutes.

Each composition, after being applied, may be set or preheated asrequired. Setting is usually effected by allowing the appliedcomposition to stand at room temperature for about 1 to about 20minutes. Preheating is usually performed at about 40 to about 120° C.,for about 1 to about 20 minutes.

The two-layer coating consisting of the colored base coating compositionand clear coating composition can be usually cured by heating at about60 to about 140° C., for about 10 to about 60 minutes. The curing ispreferably performed by heating at about 80 to about 120° C., for about10 to about 40 minutes.

In methods I and II, the colored base coating composition may be acoating composition known as a colored coating composition forapplication as a base coat over a primer coat. Preferably usable are,for example, coating compositions obtained by dissolving or dispersingin water and/or an organic solvent a base resin having carboxy, hydroxyand/or like crosslinkable functional groups, such as an acrylic resin,polyester resin, alkyd resin, urethane resin, epoxy resin or the like; acrosslinking agent, such as a polyisocyanate compound, blockedpolyisocyanate compound, melamine resin, urea resin or the like; and acoloring pigment.

Such a colored base coating composition may contain a metallic pigment,mica pigment, extender pigment, dye and/or the like, as required. Amongthese, a metallic pigment, when used, gives the resulting coating film adense, metallic appearance, and use of a mica pigment gives the coatingfilm a silky pearl tone.

In methods I and II, the clear coating composition may be a coatingcomposition known as a topcoating clear composition. Preferably usableare coating compositions obtained by dissolving or dispersing in waterand/or an organic solvent a base resin having carboxy, hydroxy and/orlike crosslinkable functional groups, such as an acrylic resin,polyester resin, alkyd resin, urethane resin, epoxy resin or the like;and a crosslinking agent, such as a polyisocyanate compound, blockedpolyisocyanate compound, melamine resin, urea resin or the like.

Such a clear coating composition may contain, as required, a coloringpigment, metallic pigment, extender pigment, dye, UV absorber and/or thelike in such an amount that does not impair the transparency.

EFFECT OF THE INVENTION

The present invention accomplishes the following remarkable effects.

(1) The uncured or cured coating layer obtained by applying the primercoating composition of the present invention to a plastic substrate hashigh conductivity (a surface electrical resistance of less than 10⁹Ω/sq.). Accordingly, other coating compositions, such as a coloredcoating composition or/and clear coating composition, can be applied onthe primer coating layer by electrostatic coating, which exhibits anexcellent deposition efficiency. Thus, the amount of toxic substancesemitted to the environment can be greatly reduced.

Further, the coating composition of the present invention can beprepared as an aqueous composition. This can reduce the amount oforganic solvent to be discharged to the environment.

(2) When the primer coating composition of the present invention isapplied to a plastic substrate and heated, the obtained cured coatinglayer has high whiteness, i.e., a lightness (L* value) of 80 or moreaccording to the L*a*b* color system defined in JIS Z 8729. Thus, theprimer coating layer has little influence on the color properties, suchas brightness, of the upper coating layers.

(3) The primer coating composition of the present invention is athermosetting composition comprising a specific resin component (a) andcrosslinking agent (b), and thus has excellent film performancecharacteristics, such as adhesion to plastic substrates, waterresistance, etc.

(4) The multilayer coating film forming method of the present inventioncan advantageously form a multilayer coating film with a lightness (Nvalue) according to the Munsell color system of 8.0 or more, and even8.3 or more, by a 3-coat 1-bake or 3-coat 2-bake process.

BEST MODE FOR CARRYING OUT THE INVENTION

Production Examples, Examples and Comparative Examples are given belowto illustrate the present invention in further detail. In the following,all parts and percentages are by weight.

PRODUCTION EXAMPLE 1 Production of White Conductive Powder

Two hundred grams of rutile titanium dioxide powder (tradename “KR-310”,manufactured by Titan Kogyo K.K., spherical particles with a meanparticle diameter of 0.3 to 0.5 μm) was dispersed in pure water toprepare 2 liters of an aqueous suspension, which was then heated to 70°C. Stannic acid solution A obtained by dissolving 69.8 g of stannicchloride (SnCl₄.5H₂O) in 500 ml of 3N hydrochloric acid, and alkalisolution B obtained by dissolving 3.3 g of sodium tungstate(Na₂WO₄.2H₂O) in 500 ml of 5N sodium hydroxide, were simultaneouslyadded dropwise to the aqueous suspension so that the aqueous suspensionhad a pH of 2 to 3. After completion of the addition, the suspension wasfiltered, and the residue was washed and dried at 110° C. for 8 hours.The dried product was heat-treated in a nitrogen gas stream (1liter/minute) at 650° C. for 1 hour, to thereby obtain white conductivepowder No. 1 comprising titanium dioxide particles whose surfaces werecovered with tin dioxide containing tungsten.

In white conductive powder No. 1, the proportion of thetungsten-containing tin dioxide covering layer was about 20 wt. %,calculated as tin dioxide, relative to the weight of titanium dioxidepigment. The proportion of tungsten in the tungsten-containing tindioxide covering layer was about 0.08 wt. % relative to the weight oftin dioxide in the covering layer.

PRODUCTION EXAMPLE 2 Production of White Conductive Powder

Two hundred grams of rutile titanium dioxide powder (tradename “KR-310”,manufactured by Titan Kogyo K.K., spherical particles with a meanparticle diameter of 0.3 to 0.5 μm) was dispersed in pure water toprepare 2 liters of an aqueous suspension, which was then heated to 70°C. To form a lower tin dioxide layer, a 5N sodium hydroxide solution,and stannic acid solution A obtained by dissolving 23.3 g of stannicchloride (SnCl₄.5H₂O) in 100 ml of 3N hydrochloric acid weresimultaneously added dropwise to the suspension so that the suspensionhad a pH of 2 to 3. After completion of the addition, the suspension wasfiltered, and the residue was washed and dried at 110° C. for 8 hours.

The obtained dry powder was dispersed in pure water to prepare 2 litersof an aqueous suspension, which was then heated to 70° C. To form anupper tin dioxide layer containing tungsten, separately prepared stannicacid solution B obtained by dissolving 69.8 g of stannic chloride(SnCl₄.5H₂O) in 600 ml of 3N hydrochloric acid, and alkali solution Cobtained by dissolving 3.3 g of sodium tungstate (Na₂WO₄.2H₂O) in 500 mlof 5N sodium hydroxide, were simultaneously added dropwise to thesuspension so that the suspension had a pH of 2 to 3. The subsequentprocedures were carried out in the same manner as in Production Example1, to thereby obtain white conductive powder No. 2 comprising titaniumdioxide particles whose surfaces were covered with a tungsten-containingtin dioxide, with an adhesive tin dioxide layer interveningtherebetween.

In white conductive powder No. 2, the proportion of tungsten-containingtin dioxide that covered the titanium oxide particles was about 30 wt.%, calculated as tin dioxide, relative to the weight of titanium dioxidepigment. The proportion of tungsten in the tungsten-containing tindioxide covering layer was about 0.08 wt. % relative to the weight oftin dioxide in the covering layer.

PRODUCTION EXAMPLE 3 Production of Chlorinated Polyolefin Resin forAqueous Coating Composition

Twelve parts of dimethylethanolamine and 5 parts of a nonionicsurfactant (tradename “Noigen EA-140”, manufactured by Daiichi KogyoYakuhin K.K.) were added to a mixture (50° C.) of 500 parts of achlorinated polypropylene (grafted with 2.0% maleic acid, chlorinecontent: 15%, saponification value: 30 mg KOH/g, weight averagemolecular weight: 80,000), 150 parts of n-heptane and 50 parts ofN-methylpyrrolidone. After stirring at the same temperature for 1 hour,2,000 parts of deionized water was gradually added, and stirring wascontinued for another hour. Subsequently, 600 parts of n-heptane anddeionized water in total was distilled off at 70° C. under reducedpressure, to thereby obtain chlorinated polyolefin emulsion No. 1 with asolids content of 24%.

PRODUCTION EXAMPLE 4 Production of Chlorinated Polyolefin Resin forAqueous Coating Composition

Using a chlorinated polypropylene (grafted with 1.9% maleic acid,chlorine content: 35%, saponification value: 28 mg KOH/g, weight averagemolecular weight: 60,000), the procedure of Production Example 3 wasfollowed to obtain chlorinated polyolefin emulsion No. 2 with a solidscontent of 24%.

PRODUCTION EXAMPLE 5 Production of Acrylic Resin Solution for AqueousCoating Composition

Forty parts of ethylene glycol monobutyl ether and 30 parts of isobutylalcohol were placed in a reaction vessel for acrylic resins equippedwith a stirrer, thermometer, reflux condenser, etc., and stirred withheating. When the mixture reached 100° C., the following monomer mixturewas added over a period of 3 hours.

styrene 10 parts methyl methacrylate 38 parts n-butyl acrylate 25 parts2-hydroxyethyl methacrylate 20 parts acrylic acid 7 parts2,2′-azobisisobutyronitril 1 part isobutyl alcohol 5 parts

After completion of the addition, the resulting mixture was maintainedat 100° C. for 30 minutes, and a mixture of 0.5 parts of2,2′-azobisisobutyronitrile and 10 parts of ethylene glycol monobutylether was added dropwise as an additional catalyst solution over aperiod of 1 hour. After continued stirring at 100° C. for one hour, themixture was cooled, and 15 parts of isobutyl alcohol was added. When themixture had cooled to 75° C., 4 parts of N,N-dimethylaminoethanol wasadded, followed by stirring for 30 minutes, to thereby obtain a solutionof hydroxy- and carboxy-containing, water-soluble acrylic resin, with asolids content of 50%. The acrylic resin had a hydroxy value of 86 mgKOH/g, an acid value of 54.5 mg KOH/g and a number average molecularweight of 20,000.

EXAMPLE 1 Production of White Conductive Primer Coating Composition ofthe Present Invention

One hundred and thirty parts of white conductive powder No. 1 was addedto 15 parts (solids content) of the acrylic resin solution obtained inProduction Example 5, and 150 parts of deionized water and 260 parts ofglass beads with a diameter of 1 mm were further added. After stirringfor 30 minutes in a shaker-type dispersing machine, the glass beads wasremoved to thereby obtain a dispersion paste.

Forty parts (solids content) of chlorinated polyolefin emulsion No. 1obtained in Production Example 3, and 30 parts (solids content) of aurethane emulsion (tradename “Sanprene UX-5100A”, manufactured by SanyoChemical Industries, Ltd.) were added to the dispersion paste, followedby thorough agitation in a mixer with agitating elements (tradename “TKPipeline Homo Mixer Model SL”, manufactured by Tokushu Kika Kogyo Co.,Ltd., agitating element diameter: 40 mm). Further, immediately prior toapplication, 15 parts (solids content) of a hydrophilized isocyanurateof hexamethylene diisocyanate (tradename “Bayhydur 3100”, manufacturedby Sumika Bayer Urethane Co., Ltd.) was added, the mixture wasthoroughly agitated in the mixer with agitating elements to adjust theviscosity to 15 seconds/Ford cup #4/20° C., thus giving aqueous whiteconductive primer coating No. 1.

EXAMPLES 2 TO 4 Production of White Conductive Primer CoatingCompositions of the Present Invention

Following the procedure of Example 1 and using the components shown inTable 1 in the amounts indicated, aqueous white conductive primercoating compositions No. 2 to No. 4 were obtained.

Table 1 shows the amounts of the components of white conductive primercoating compositions of Examples 1 to 4.

TABLE 1 Example 1 2 3 4 White conductive primer coating No. 1 No. 2 No.3 No. 4 composition Chlorinated polyolefin emulsion No. 1 40 40Chlorinated polyolefin emulsion No. 2 40 40 Acrylic resin of ProductionExample 5 15 15 15 15 Urethane emulsion (*1) 30 30 30 30 Hydrophilizedisocyanurate of 15 15 15 15 hexamethylene diisocyanate (*2) Whiteconductive powder No. 1 130 100 White conductive powder No. 2 150 80Rutile titanium dioxide (*3) 30 50

All the amounts in Table 1 are parts by weight of solids contents.

In Table 1, (*1) to (*3) indicate the following.

-   (*1) Urethane emulsion: tradename “Sanprene UX-5100A”, manufactured    by Sanyo Chemical Industries, Ltd.-   (*2) Hydrophilized isocyanurate of hexamethylene diisocyanate:    tradename “Bayhydur 3100”, manufactured by Sumika Bayer Urethane    Co., Ltd.-   (*3) Rutile titanium dioxide: tradename “JR-903”, manufactured by    TAYCA CORP., spherical particles with a mean particle diameter of    0.4 μm

COMPARATIVE EXAMPLES 1 TO 5 Production of Comparative White ConductivePrimer Coating Compositions

Aqueous white conductive primer coating compositions No. 5 to No. 9 wereobtained by using the components shown in Table 2 in the amountsindicated, and following the procedure of Example 1.

In Comparative Examples 2 and 3, however, the resin and pigmentcomponents were mixed using a mixer with agitating elements in place ofthe shaker-type dispersing machine, so as to maintain the shape of theneedlelike titanium dioxide whose surfaces were covered with tindioxide/antimony or metal oxide-covered flaky mica.

Table 2 shows the amounts of the components of the white conductiveprimer coating compositions of Comparative Examples 1 to 5.

TABLE 2 Comparative Example 1 2 3 4 5 White conductive primer coatingNo. 5 No. 6 No. 7 No. 8 No. 9 composition Chlorinated polyolefin 40 4040 emulsion No. 1 Chlorinated polyolefin 40 40 emulsion No. 2 Acrylicresin of Production 15 15 15 15 15 Example 5 Urethane emulsion (*1) 3030 30 30 30 Hydrophilized isocyanurate of 15 15 15 15 15 hexamethylenediisocyanate Conductive carbon (*4) 2.5 2.5 Needlelike titanium dioxidewhose 100 surfaces were covered with tin dioxide/antimony Metaloxide-covered flaky mica (*6) 3 Rutile titanium dioxide (*3) 130 130 130130 130

All the amounts in Table 2 are parts by weight of solids contents.

In Table 2, (*1) to (*3) indicate the same products as shown above. (*4)to (*6) indicate the following.

-   (*4) Conductive carbon: tradename “Ketjenblack EC600J”, manufactured    by Lion Corp.-   (*5) Needlelike titanium dioxide whose surfaces were covered with    tin dioxide/antimony: tradename “Dentall WK500”, manufactured by    Otsuka Chemical Co., Ltd.-   (*6) Metal oxide-covered flaky mica: tradename “Iriogin 103R”,    manufactured by Merck Ltd., a non-conductive filler comprising flaky    mica covered with a SnO₂ lower layer and a TiO₂ upper layer formed    over the lower layer, mean particle diameter: 22 μm

EXAMPLES 5 TO 9 AND COMPARATIVE EXAMPLES 6 TO 10 Formation of MultilayerCoating Films

Using white conductive primer coating compositions No. 1 to No. 4 of thepresent invention obtained in Examples 1 to 4, multilayer coating filmsof Examples 5 to 8 were formed by a 3-coat 2-bake process comprisingcoating steps 1 and 2 described below. Separately, using primer coatingcomposition No. 3 of the present invention obtained in Example 3, amultilayer coating film of Example 9 was formed by a 3-coat 1-bakeprocess comprising coating steps 1 and 2, in which, however, the primercoating layer was not cured in coating step 1.

Further, using comparative white primer coating compositions No. 5 toNo. 9 obtained in Comparative Examples 1 to 5, multilayer coating filmsof Comparative Examples 6 to 10 were formed by a 3-coat 2-bake processcomprising coating steps 1 and 2.

Coating Step 1: Application of White Conductive Primer Coating Layer

Black polypropylene was molded into bumper shapes, degreased, and usedas plastic substrates. White conductive primer coating compositions No.1 to No. 9 were applied to the substrates by air spraying to a thicknessof 20 μm (when cured). The applied coating layers were allowed to standat room temperature for 2 minutes, preheated at 80° C. for 3 minutes,and cured by heating at 120° C. for 20 minutes. The L* value and surfaceelectrical resistance A of the cured coating layers were measured by themethods described hereinbelow.

Coating Step 2: Formation of Multilayer Coating Film Comprising AqueousColored Base Coating Composition and Organic Solvent-Based Clear CoatingComposition Applied on the White Conductive Primer Coating Layer

An aqueous thermosetting translucent colored base coating composition(tradename “WBC-710 Mica Base”, manufactured by Kansai Paint Co., Ltd.)was electrostatically applied to the cured coating layer obtained incoating step 1 to a thickness of 15 to 20 μm (when cured) and preheatedat 80° C. for 3 minutes. The surface electrical resistance B was thenmeasured by the method described hereinbelow. Subsequently, an organicsolvent-based acrylic resin/urethane resin thermosetting clear coatingcomposition (tradename “Soflex #520 Clear”, manufactured by Kansai PaintCo., Ltd.) was applied to the uncured colored coating layer to athickness of 25 μm (when cured), allowed to stand at the roomtemperature for 5 minutes, and heated at 120° C. for 30 minutes tosimultaneously cure the colored coating layer and clear coating layer,thereby giving a multilayer coating film.

The L* value, surface electrical resistance A and surface electricalresistance B were measured by the following methods.

L* value: The coating layer of the white conductive primer coatingcomposition was cured by heating at 120° C. for 20 minutes. The L* valueaccording to the L*a*b* color system defined in JIS Z 8729 of the curedcoating layer was measured using a colorimeter (tradename “ColorComputer SM-7”, manufactured by Suga Test Instruments Co., Ltd.).

Surface electrical resistance A: A white conductive primer coatingcomposition was applied and cured by heating, and the surface electricalresistance of the cured coating layer was measured using an electricalresistance meter (manufactured by TREK, tradename “MODEL 150”). When themeasured value is less than 10⁹ Ω/sq., a colored base coating can beelectrically applied on the cured coating layer.

Surface electrical resistance B: After applying and curing by heating awhite conductive primer coating composition, a portion of the curedcoating layer was masked so that the portion would be able to be broughtinto contact with terminals for measuring the surface electricalresistance after applying a base coating composition to the cured primercoating layer. Subsequently, an aqueous thermosetting translucentcolored base coating composition (tradename “WBC-710 Mica Base”,manufactured by Kansai Paint Co., Ltd.) was electrostatically appliedand preheated at 80° C. for 3 minutes, and the surface electricalresistance of the primer coating layer was measured using an electricalresistance meter (manufactured by TREK, tradename “MODEL 150”). When themeasured value is less than 10⁹ Ω/sq., a clear coating can be furtherapplied electrostatically.

Table 3 shows the test results of the multilayer coating films ofExamples 5 to 9 formed using the primer coating compositions of thepresent invention.

TABLE 3 Example 5 6 7 8 9 White conductive primer No. 1 No. 2 No. 3 No.4 No. 3 coating composition L* value 82 81 86 88 — Surface electrical 1× 10⁷ 1 × 10⁷ 1 × 10⁸ 7 × 10⁸ 1 × 10⁷ resistance A (Ω/sq.) Surfaceelectrical 3 × 10⁷ 1 × 10⁷ 3 × 10⁸ 7 × 10⁸ 3 × 10⁷ resistance B (Ω/sq.)

Table 4 shows the test results of the multilayer coating films ofComparative Examples 6 to 10 formed using the comparative primer coatingcompositions.

TABLE 4 Comparative Example 6 7 8 9 10 White conductive No. 5 No. 6 No.7 No. 8 No. 9 primer coating composition L* value 60 78 80 50 93 Surfaceelectrical 3 × 10⁶ 1 × 10⁶ 1 × 10¹³ 1 × 10⁸ 1 × 10¹³ resistance A(Ω/sq.) Surface electrical 3 × 10⁷ 1 × 10⁷ — 1 × 10⁸ — resistance B(Ω/sq.)

In Table 4, the surface electrical resistance B of the multilayercoating films of Comparative Examples 8 and 10 was not measurable. Thisis because, in coating step 2, the colored coating composition and clearcoating composition were not able to be electrostatically applied to thecoating layers of comparative white conductive primer coatingcompositions No. 7 and No. 9, since the coating layers had a surfaceelectrical resistance as high as 1×10¹³ Ω/sq.

The multilayer coating films obtained in Examples 5 to 9 and ComparativeExamples 6, 7 and 9 were tested, by the following methods, for filmappearance, N value according to the Munsell color system defined in JISZ 8721, film performance (adhesion and water resistance), andrecyclability.

Film appearance: The perpendicular portion of the coated substrate waschecked for abnormalities (sagging, after tack and blistering) of thecoating film by the naked eye, and evaluated by the following criteria.

A: none of the above abnormalities was observed; B: one or more of theabnormalities (sagging, after tack and blistering) were observed; C: oneor more of the abnormalities (sagging, after tack and blistering) wereremarkable.

N value according to the Munsell color system defined in JIS Z 8721: TheN value in the Munsell chart of the three-layer coating film consistingof the primer coating layer, colored coating layer and clear coatinglayer was determined. 0 is black and 10 is pure white.

Adhesion: In the multilayer coating film consisting of three layers,i.e., the primer coating layer, colored coating layer and clear coatinglayer, cuts reaching the substrate were made with a cutter so as to form100 squares of a width of 2 mm, and an adhesive tape was adhered to thecut surface, and rapidly peeled off at 20° C. The number of squaresremaining was counted and evaluated according to the following criteria.

A: all the squares remained, good adhesion; B: 90 to 99 squaresremained, somewhat poor adhesion; C: less than 90 squares remained; pooradhesion.

Water resistance: The multilayer coating film consisting of threelayers, i.e., the primer coating layer, colored coating layer and clearcoating layer, was immersed in warm water at 40° C. for 240 hours, andcuts reaching the substrate were made in the coating film with a cutterso as to form 100 squares of a width of 2 mm. An adhesive tape was thenadhered to the cut surface and rapidly peeled off at 20° C. The numberof squares remaining was counted and evaluated according to thefollowing criteria.

A: all the squares remained, good water resistance; B: 90 to 99 squaresremained, somewhat poor water resistance; C: less than 90 squaresremained, poor water resistance.

Recyclability: The substrate with the multilayer coating film consistingof three layers (the primer coating layer, colored coating layer andclear coating layer) was ground using a grinder into particles with adiameter of about 1 mm or less, and the ground product was observed withan optical microscope. The ground product was checked for conductivefiller particles with an aspect ratio of 3 or more, which have adverseeffects on the human body. The recyclability was evaluated according tothe following criteria.

A: the conductive filler particles contained in the ground product hadspherical shapes with an aspect ratio of less than 3, and thus thecoated plastic substrate had excellent recyclability;

B: the ground product contained conductive filler particles with anaspect ratio of 3 or more, and thus the coated plastic substrate hadpoor recyclability.

Table 5 shows the results of the performance test of the multilayercoating films.

TABLE 5 Comparative Example Example 5 6 7 8 9 6 7 9 Film A A A A A C B Aappearance N value 8.3 8.0 8.7 8.9 8.7 6.0 8.5 6.0 Adhesion A A A A A BB A Water A A A A A C C A resistance Recyclability A A A A A A C A

1. A white conductive primer coating composition comprising: (a) 100parts by total weight of a chlorinated polyolefin resin with a chlorinecontent of 10 to 40 wt. %, and at least one modifier resin selected fromthe group consisting of acrylic resins, polyester resins andpolyurethane resins; (b) 5 to 50 parts by weight of a crosslinkingagent; and (c) 10 to 200 parts by weight of a white conductive powdercomprising white inorganic pigment particles having on their surfaces atin dioxide covering layer containing tungsten; in the white conductivepowder (c), the proportion of the tungsten-containing tin dioxidecovering layer is 3 to 150 wt. % on a tin dioxide basis, relative to theweight of white inorganic pigment; the composition being capable offorming a coating with a lightness (L* value) of 80 or more asdetermined according to the L*a*b* color system defined in JIS Z 8729,by being applied to a plastic substrate and cured by heating; and thecomposition, when formed into an uncured or cured coating applied on aplastic substrate, having a surface electrical resistance of less than10⁹ Ω/sq.
 2. A primer coating composition according to claim 1, wherein,in component (a), the proportions of the chlorinated polyolefin resinand modifier resin are 10 to 90 wt. % and 90 to 10 wt. %, respectively,of the total of these resins.
 3. A primer coating composition accordingto claim 1, wherein the white conductive powder (c) comprises whiteinorganic pigment particles having a tin dioxide layer on their surfacesand a tin dioxide covering layer containing tungsten on the tin dioxidelayer.
 4. A primer coating composition according to claim 1, wherein, inthe tungsten-containing tin dioxide covering layer in the whiteconductive powder (c), the proportion of tungsten is 0.1 to 20 wt. %relative to the weight of tin dioxide.
 5. A primer coating compositionaccording to claim 1, further comprising (d) up to 200 parts by weightof a rutile titanium dioxide.
 6. A primer coating composition accordingto claim 1, which is an aqueous coating composition.
 7. A 3-coat 1-bakemethod of forming a multilayer coating film, the method comprising thesteps of: (1) applying a white conductive primer coating compositionaccording to claim 1 to a plastic substrate; (2) electrostaticallyapplying a colored base coating composition on the uncured coating layerof the primer coating composition; (3) electrostatically applying aclear coating composition on the uncured coating layer of the basecoating composition; and then (4) curing by heating the three-layercoating comprising the primer coating composition, colored base coatingcomposition and clear coating composition.
 8. A 3-coat 2-bake method offorming a multilayer coating film, the method comprising the steps of:(1) applying a white conductive primer coating composition according toclaim 1 to a plastic substrate, followed by curing by heating; (2)electrostatically applying a colored base coating composition on thecured coating layer of the primer coating composition; (3)electrostatically applying a clear coating composition on the uncuredcoating layer of the base coating composition; and then (4) curing byheating the two-layer coating comprising the colored base coatingcomposition and clear coating composition.